(1) Field of the Invention
The present invention relates to a method for exhaust gas purification, capable of effectively purifying harmful substances present in the exhaust gas emitted from an internal combustion engine, particularly hydrocarbons (HC) generated in a large amount during the cold start of the engine, without adversely affecting the purification of NOx also present in the exhaust gas, as well as to a system for exhaust gas purification used in the method.
(2) Description of Related Art
In order to activate the catalyst for exhaust gas purification provided in an exhaust pipe of an engine, at an early period during the cold start of the engine, there is known a technique of introducing secondary air into the exhaust pipe at a site of the pipe upstream of the catalyst in the flow direction of an exhaust gas passing through the pipe. During the cold start of an engine when the ratio of fuel to air is high (fuel rich), a combustion reaction by catalyst hardly takes place and, moreover, the heating of catalyst by the heat generated by the reaction is hardly expected; hence, in the above technique, it is hoped to, by introducing secondary air as mentioned above, adjust the composition of exhaust gas to a lean side (oxygen excessive) and promote the activation (start of combustion reaction) of catalyst.
There is also known a technique of (1) using a system provided in an exhaust pipe of an engine, comprising a catalyst and an adsorbent containing an adsorbent component (e.g. zeolite) capable of adsorbing hydrocarbons (HC) and (2) introducing secondary air into the exhaust pipe. In this technique, the adsorbent is used to adsorb and store the unburnt HC discharged from the engine in a large amount during the cold start, temporarily until the catalyst is activated.
As an example of the above technique, there is disclosed, in Japanese Patent Application Laid-Open No. 185343/-1994, a method for exhaust gas purification, which method uses a system for exhaust gas purification provided in an exhaust pipe of an internal combustion engine, comprising (1) an adsorbent containing an adsorbent component capable of adsorbing harmful substances (e.g. HC) present in the exhaust gas emitted from the engine and (2) a catalyst containing a catalyst component capable of reducing harmful substances present in the exhaust gas, which method comprises introducing an oxidizing gas (e.g. secondary air) into the exhaust gas for a certain period of the time span in which the HC., etc. present in the exhaust gas generated during the cold start of the internal combustion engine are adsorbed by the adsorbent and then are desorbed from the adsorbent with the temperature rise of the adsorbent brought about by the heat of exhaust gas.
In such a system for exhaust gas purification containing an adsorbent, as the adsorbent temperature rises owing to the heat of exhaust gas and the desorption of HC from adsorbent begins, the HC concentration in exhaust gas rises temporarily; therefore, even after the activation of catalyst, introduction of secondary air is continued until the desorption of HC from adsorbent is completed, in order to promote the purification (combustion) of the desorbed HC.
As a result, in a system using an adsorbent, as compared with a system using a catalyst alone, secondary air must be introduced for a long time. Since nitrogen oxides (NOx), which are also harmful substances present in exhaust gas, are hardly purified in a lean atmosphere even in the presence of a catalyst, the above conventional technique has had a problem in that the NOx level in exhaust gas is not reduced during the period of secondary air introduction. Thus, in the above conventional technique, although HC can be purified effectively during the cold start of the engine, NOx purification during the cold start has been insufficient.